This invention relates to photographic objectives having a mechanism for preventing the change of angular field of view during focusing, and, more particularly, to a zoom lens having the same mechanism.
One known form of focusing system of a photographic objective to arrange a movable lens group in front of a stationary lens group, or groups.
Particularly in the case of zoom lenses, because of their requirement of giving to the zoom part, consisting of a variator and a compensator, an object point (an image point of the focusing lens) at an always constant position independently of the object distance so that a constant image plane is maintained during zooming. For the focusing system, use is made of the so-called front-focusing arrangement.
A serious problem encountered at the present state of art of the focusing provision made in photographic objectives and particularly zoom lens systems is that the change of angular field of view in the object space during focusing should be either avoided, or at least minimized. Otherwise, this change in angular field results in the perplexing impression that zooming is taking place. This is undesirable to the visual sense.
In order to minimize the change in angular field during focusing, various focusing systems have been proposed. According to the proposal in U.S. Pat. No. 3,682,534, the focusing system or front lens group is constructed by two components both of which have negative refractive powers, and of which the frontmost, or the first is stationary during focusing, and only the second is made movable for focusing, thus minimizing the change of angular field. According to another proposal in U.S. Pat. No. 4,099,845, the front lens group is constructed by three components of negative, positive and positive powers, of which only the second is moved rearwardly as focusing takes place down to shorter object distances, thus obtaining a focusing system with advantage to reduce variation of the angular field. It is noticed that the provision of these systems primarily aims at achieving minimization of the diameter of the front lens member and shortening the object distance to which focusing can be effected, and that the reduction of variation of the angular field may be considered to result as a secondary product.
According to still another proposal of Japanese Patent Laid-Open Application Sho 53-6051, the front lens group is divided into two parts which are moved in certain differential relation to each other to effect focusing, thereby it being made possible to reduce the variation of the angular field to zero.
However, the above cited U.S. Pat. Nos. 3,682,532 and 4,099,845 though having allowed for the reduction of the variation of angular field still leave too great a residual to accept, and the above-cited Japanese Patent Laid-Open Application Sho 53-6051, though having achieved perfect stabilization against the change of angular field, has a disadvantage arising from the requirement of establishing the relationship in which the two focusing members are moved. This results in that the focusing control mechanism is made very complicated. Moreover, it has now been found in the latter connection that when applied to an objective lens system where the point of position of the exit pupil is largely spaced from the rear principal point of the entire system, an appreciable error is produced.